The present invention relates to the field of devices and methods for assisting hearing in persons and particularly to the field of transducers for producing vibrations in the inner ear.
The seemingly simple act of hearing is a thing that can easily be taken for granted. Although it seems to us as humans we exert no effort to hear the sounds around us, from a physiologic standpoint, hearing is an awesome undertaking. The hearing mechanism is a complex system of levers, membranes, fluid reservoirs, neurons and hair cells which must all work together in order to deliver nervous stimuli to the brain where this information is compiled into the higher level perception we think of as sound.
As the human hearing system encompasses a complicated mix of acoustic, mechanical and neurological systems, there is ample opportunity for something to go wrong. Unfortunately this is often the case. It is estimated that one out of every ten people suffer some form of hearing loss. Surprisingly, many patients who suffer from hearing loss take no action in the form of treatment for the condition. In many ways hearing is becoming more important as the pace of life and decision making increases as we move toward an information based society. Unfortunately for the hearing impaired, success in many professional and social situations may be becoming more dependent on effective hearing.
Participants in the field of Hearing Science are well aware of the advances that are being made to help combat hearing loss and further scientific understanding of hearings processes. Several key ongoing projects have been instrumental in demonstrating the potential for advanced devices to help the hearing impaired. Although no one can argue that conventional acoustic hearing devices have not been helpful to many of the hearing impaired, the majority of the world wide impaired population, for whatever reason, has rejected their use. Hopefully, as advancements are made and alternatives to today's conventional devices appear, the number of hearing impaired patients getting the help they need will hopefully improve.
The first hearing device first appeared in Roman times and consisted of a hollow dome "catch" that probably provided about 15-20 decibels of sound amplification for the user. Ear Trumpets and conversation tubes were widely available in the 1700 and 1800's and the first electronic hearing aids began making their debut in the early 1900's. The development of the transistor lead to smaller more power efficient aids that began to appear in the 1950's. Interestingly, transistor type hearing aids were Sony Corporation's first product before they advanced into audio equipment. In fact, many inventions, including the telephone were hearing aid development spin offs.
In the 1960's and 70's the hearing arena underwent a period of accelerated development. Hearing device companies and product lines began multiplying rapidly. Measurement standards for prescribing devices, patient hearing evaluation and manufacturing standards for hearing devices were becoming more established. Audiologists were working to advance device technology, continuing hearing research and instituting improvements in hearing device measurement and fitting technology. Audiologic advancements in hearing assessment and diagnosis of hearing disorders translated into better diagnosis and treatment for the hearing impaired. State regulation of the dispensing industry through licensing and certification programs was developed to insure quality of hearing aid dispensing practices.
The hearing impaired patient in 1995 has a wide variety of hearing devices to choose from. Devices that have improved circuits, enhanced fitting parameters that allow the electronics to be customized to the patients individual hearing loss (i.e., similar to an eye glass prescription, one size does not fit all). New devices located completely in the patients ear canal are available that are cosmetically superior to the large bulky devices of years past and can be virtually invisible. Many manufacturers participate in the hearing marketplace which is a sizable 3 billion dollar worldwide market.
A number of auditory system defects are known to impair or prevent hearing. To illustrate such defects, a schematic representation of part of the human auditory system is shown in FIG. 1. The auditory system is generally comprised of an external ear AA, a middle ear JJ, and an internal ear FF. The external ear AA includes the ear canal BB and the tympanic membrane CC, and the internal ear FF includes an oval window EE and a vestibule GG which is a passageway to the cochlea (not shown). The middle ear JJ is positioned between the external ear and the middle ear, and includes an eustachian tube KK and three bones called ossicles DD. The three ossicles DD: the malleus LL, the incus MM, and the stapes HH, are positioned between and connected to the tympanic membrane CC and the oval window EE.
In a person with normal hearing, sound enters the external ear AA where it is slightly amplified by the resonant characteristics of the ear canal BB. The sound waves produce vibrations in the tympanic membrane CC, part of the external ear that is positioned at the distal end of the ear canal BB. The force of these vibrations is magnified by the ossicles DD.
Upon vibration of the ossicles DD, the oval window EE, which is part of the internal ear FF, conducts the vibrations to cochlear fluid (not shown) in the inner ear FF thereby stimulating receptor cells, or hairs, within the cochlea (not shown). Vibrations in the cochlear fluid also conduct vibrations to the round window (not shown). In response to the stimulation, the hairs generate an electrochemical signal which is delivered to the brain via one of the cranial nerves and which causes the brain to perceive sound.
The vibratory structures of the ear include the tympanic membrane, ossicles (malleus, incus, and stapes), oval window, round window, and cochlea. Each of the vibratory structures of the ear vibrates to some degree when a person with normal hearing hears sound waves. However, hearing loss in a person may be evidenced by one or more vibratory structures vibrating less than normal or not at all.
Some patients with hearing loss have ossicles that lack the resiliency necessary to increase the force of vibrations to a level that will adequately stimulate the receptor cells in the cochlea. Other patients have ossicles that are broken, and which therefore do not conduct sound vibrations to the oval window.
Prostheses for ossicular reconstruction are sometimes implanted in patients who have partially or completely broken ossicles. These prostheses are designed to fit snugly between the tympanic membrane CC and the oval window EE or stapes HH. The close fit holds the implants in place, although gelfoam is sometimes packed into the middle ear to guard against loosening. Two basic forms are available: total ossicular replacement prostheses which are connected between the tympanic membrane CC and the oval window EE; and partial ossicular replacement prostheses which are positioned between the tympanic membrane and the stapes HH. Although these prostheses provide a mechanism by which vibrations may be conducted through the middle ear to the oval window of the inner ear, additional devices are frequently necessary to ensure that vibrations are delivered to the inner ear with sufficient force to produce high quality sound perception.
Various types of hearing aids have been developed to restore or improve hearing for the hearing impaired. With conventional hearing aids, sound is detected by a microphone, amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or another type of transducer into the middle ear by way of the tympanic membrane. Often the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion.
Attempts have been made to eliminate the feedback and distortion problems associated with conventional hearing aid systems. These attempts have yielded devices which convert sound waves into electromagnetic fields having the same frequencies as the sound waves. A microphone detects the sound waves, which are both amplified and converted to an electrical current. A coil winding is held stationary by being attached to a nonvibrating structure within the middle ear. The current is delivered to the coil to generate an electromagnetic field. A magnet is attached to an ossicle within the middle ear so that the magnetic field of the magnet interacts with the magnetic field of the coil. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear.
Existing electromagnetic transducers present several problems. Many are installed using complex surgical procedures which present the usual risks associated with major surgery and which also require disarticulating (disconnecting) one or more of the bones of the middle ear. Disarticulation deprives the patient of any residual hearing he or she may have had prior to surgery, placing the patient in a worsened position if the implanted device is later found to be ineffective in improving the patient's hearing.
Existing devices also are incapable of producing vibrations in the middle ear which are substantially linear in relation to the current being conducted to the coil. Thus, the sound produced by these devices includes significant distortion because the vibrations conducted to the inner ear do not precisely correspond to the sound waves detected by the microphone.
An improved transducer is therefore needed which will ultimately produce vibrations in the cochlea that have sufficient force to stimulate hearing perception with minimal distortion.